Commercial and utility-scale photovoltaic (“PV”) power systems (“PV systems”) require a significant initial investment and ongoing maintenance effort in order to meet their performance and financial expectations over the lifetime of the system. Accordingly, it is important to accurately and timely monitor key parameters of the PV system to evaluate the performance of the PV system. By monitoring PV systems, component failures and losses caused by factors that negatively affect the efficiency of the PV system, such as soiling of solar panels, may be identified so that a financial value of the loss may be determined.
Monitoring systems for PV systems are known and described in U.S. Pat. Nos. 6,425,248, 7,336,201, 7,742,897, 8,725,437, 8,738,328, 9,660,574, U.S. Patent Application Publication No. 2016/0190984, and U.S. Patent Application Publication No. 2015/0012258 which are each incorporated herein by reference in their entirety. PV monitoring systems typically use data from sensors, such as pyranometers, reference cells, and other types of irradiance sensors, to measure sunlight received at one or more locations of a PV system. These irradiance sensors are integral to the analysis commonly utilized to maintain optimal performance in a solar PV system. Data from irradiance sensors may be used to determine a cause of a loss. For example, some PV monitoring systems can use data from irradiance sensors to distinguish between a transitory loss (such as cloud cover or snow accumulation at the PV system) and a loss which requires some action at the PV system (such as a hardware failure or soiling of the PV system).
The irradiance data can also be used to assign a financial value the loss. The financial value of the loss may further be used to determine if it is economical to perform corrective action to ensure that the PV system is operating efficiently. For some losses, it may not be economical to perform corrective action because the value of the lost energy is less than the cost of the corrective action. Determining the financial value of the loss becomes increasingly vital in remotely located PV systems where it may not be cost effective to send technicians to take action to mitigate a small loss. Accordingly, obtaining accurate measurements of irradiance at a PV system is very important to the operation and monitoring of the PV system, for correctly identifying the cause of a loss, and quantifying the value of a loss.
Unfortunately, the accuracy of irradiance sensors may deteriorate over time for a variety of reasons. For example, accuracy of irradiance sensors may be negatively influenced by the accumulation of dirt. Irradiance sensors must also be substantially level to provide accurate readings. However, irradiance sensors frequently move out of level over time decreasing the accuracy of data collected. Further, on-site irradiance sensors, such as pyranometers, that provide irradiance data for PV systems are known to become less accurate over time due to deterioration and equipment failure. Some known pyranometers decrease in accuracy by 3 percent or more annually. Accordingly, irradiance sensors require frequent on-site maintenance or calibration by the manufacturer. It is not uncommon to return irradiance sensors to their manufacturer every 1 or 2 years.
The calibration of irradiance sensors often requires removal of the sensor and re-installation after it has been calibrated. Occasionally, irradiance sensors that are out of calibration by greater than a predetermined amount may be replaced. Sending technicians to inspect, calibrate, and/or replace irradiance sensors frequently requires a substantial cost in labor because PV systems are often remotely located.
Some methods of calibrating irradiance sensors include comparing the performance of the irradiance sensor to a reference irradiance sensor in an indoor facility. Some suppliers of irradiance sensors recommend factory calibration at least every two years. One manufacturer claims calibration can usually be completed within four weeks, although urgent calibration can be performed in three weeks or less. As one of skill in the art will appreciate, the performance of PV systems cannot be accurately monitored without an irradiance sensor for such a long period of time. Accordingly, a temporary irradiance sensor must frequently be installed at the PV system during calibration of an irradiance sensor, further increasing the cost and labor expense associated with maintaining the irradiance sensor. Of course, after a recalibrated irradiance sensor is returned from a factory calibration, a technician must make another trip to a remote PV system to remove the temporary irradiance sensor and reinstall the recalibrated irradiance sensor, incurring more labor and travel expenses. As solar PV systems are generally expected to operate for 20 years or more, the calibration of irradiance sensors requires considerable labor and associated expense during the operational life of the PV system, especially when the PV system is located in a remote area.
Due to the known deficiencies and inherent deterioration of the accuracy of irradiance sensors, some PV system operators install additional irradiance sensors at PV system. Other operators of PV systems schedule periodic on-site maintenance and/or replacement of their irradiance sensors. As can be appreciated, both of these actions further increase the expense associated with a PV system.
Thus, there is a need for a system and method that can analyze the performance of an irradiance sensor and compensate for drift of the irradiance sensor while requiring minimal, or no, capital and equipment investment. The system and methods of the present invention can also adjust a report system to prevent unnecessary reports or alarms for an irradiance sensor which is out of specification but which provides irradiance data which may be corrected by use of a correction factor.